Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is well suited for application of characters onto a variety of surfaces including porous and non-porous surfaces.
Reviews of various aspects of ink jet printing can be found in these publications: Kuhn et al., Scientific American, April, 1979, 162-178; and Keeling, Phys. Technol., 12(5), 196-303 (1981). Various ink jet apparatuses are described in the following U.S. Pat. Nos. 3,060,429, 3,298,030, 3,373,437, 3,416,153, and 3,673,601.
Pigmented ink, which includes insoluble pigment particles, is frequently used in ink jet printing. Although it has a number of desirable characteristics, pigmented ink also has a significant drawback. The pigment particles tend to agglomerate and settle at the bottom surface of the ink supply container, causing nozzle clogging and disruption in printing, as well as a decrease in print contrast. The nozzles typically have a diameter around about 2.5 to 3.0 thousandths of an inch.
Attempts have been made to maintain pigment particles uniformly suspended in the ink jet ink. However, many of these attempts have not been successful or provide techniques having drawbacks. For example, one technique involves the use of a magnetic stirrer disposed in the ink container. The rod-shaped magnetic stirrer, usually disposed at the bottom of the container, is driven by a varying magnetic field generated by rotating magnets outside of the container.
Use of this technique in ink jet printing, however, is costly and complex. The system occupies a large volume of printer space. In addition, the system has a relatively short service life, because it involves moving mechanical parts causing significant mechanical wear. Further, it is time-consuming to extract the magnetic rod from a depleted ink container and deposit the magnetic rod in a full container every time a depleted ink container is replaced by a new ink container.
The foregoing indicates that there exists a need for a system that reduces or eliminates pigment settling. There further exists a need for an ink jet printing system that prints without nozzle clogging for extended periods of time.
These and other advantages of the present invention as well as additional inventive features will be apparent from the detailed description of the invention set forth herein below.